Analog cellular systems use frequency modulation (FM) to transmit information which allows the use of cost-effective receivers employing limiting and frequency discrimination. With the advent of digital cellular systems, receiver designs are complicated by the need to have a wide dynamic range, linear receiver with channel equalization to attain the highest level of performance. This typically requires more costly implementations than receivers used in analog cellular systems.
In a time division multiple access (TDMA) digital cellular system, traffic and signalling information is transmitted in bursts (normal bursts) during timeslots of periodic intervals. Multipath fading will introduce transmission errors in some of these bursts, which is mitigated through the use of interleaving and coding. These bursted error events can most effectively be dealt with by applying soft decision information to the decoding process. To generate soft decision information, the variations in received amplitude should ideally be preserved for each interleaved burst to assist in channel equalization and subsequent decoding. This is typically accomplished through the use of a linear receiver covering the full faded dynamic range.
When a mobile requires access to a TDMA digital cellular system, it transmits an access burst to the system to request such access. The duration of the access burst is shortened relative to the normal bursts (traffic and signalling) to compensate for the time delay between a mobile's transmission of the access burst and a base-station's reception of the access burst. The access burst is a one-time event and, thus, the data need not be (and is not) interleaved. The access burst coding does, however, provide some means of combating fading.
Typical receiver designs in TDMA digital cellular systems implement common automatic gain control (AGC) elements for all burst-types. AGC on the access burst is treated differently from the normal bursts in the linear receiver by performing a fast AGC operation on the access burst. In this configuration, the linear receiver must determine when a mobile is on channel, estimate its signal strength, and set/hold the AGC for the remainder of the access burst. This method requires a very fast signal strength indicator (SSI) circuit and a threshold indicator to sense the presence of a mobile attempting access to the system. AGC on normal bursts is done by using an algorithm which employs a long term averaging component and a short term signal strength component. In both cases, as stated above, common AGC elements are used and their settings held through the timeslot.
The fast AGC method described above has several limitations: (1) memory is required to store values which map the raw signal strength indication to a gain setting, (2) the SSI must be very fast to track the mobile transmission's turn-on characteristic, and (3) there is some probability that an incorrect AGC setting will be made during the access burst. This is probable because the longer a mobile's transmission is delayed, the greater the likelihood that a noise burst or an interfering signal will cause a false detection of a mobile's presence. If the AGC setting is based on the signal strength of this noise or interference, a delayed mobile transmission may be received later in the timeslot which is well outside the A/D converter's window. In rural installations which may have extended cells up to 120 km, two adjacent timeslots could be used for the access burst. In this case, there is a greater probability of a false AGC setting due to a noise burst or an interfering signal crossing the presence threshold.
Thus, a need exists for a receiver architecture which is capable of receiving access, traffic and signalling bursts while overcoming the above described limitations.